One of the busiest U.S. travel days of the year also looks to be the most difficult, as bad weather in the northeastern states has already forced mass flight cancellations.

As of 2:15 p.m. ET Wednesday, FlightAware reported 596 flights into, out of or within the United States had been canceled ahead of a storm expected to bring rain and snow from Washington to Boston. Over 2,500 flights have been delayed, the site said.

Around one-third of those flights were meant to have originated at either New Jersey's Newark or New York's LaGuardia airports. By 2:15 p.m. ET flights into those two airports were delayed an average of more than three hours. New York's John F. Kennedy International Airport reported average delays of an hour and a half for inbound flights.

Philadelphia International Airport, also set to see the effects of Winter Storm Cato, saw average inbound flight delays of an hour and 43 minutes by that time.

"The delays will begin accumulating early afternoon as the worst of the weather impacts the Northeast and will continue to cascade through the evening," FlightAware CEO Daniel Baker said in a statement.

West Point Cadet Jameson Albers, 21, originally had a 5:30 p.m. flight home to Nashville from LaGuardia Airport. "I bumped my flight to an 11:30 (a.m.) flight to try to get in before the really bad weather hits," he told the Associated Press.

In addition to operating on a weekday schedule, NJ Transit said additional "early getaway" service will be available from New York, Newark and Hoboken Terminal starting Wednesday afternoon.

Several major airlines dropped their ticket-change fees for people flying in and out of the Northeast, allowing passengers to try to sneak on an earlier flight, though that appeared to be a challenging proposition since most planes were filled.

United said it was planning to cancel 100 flights Wednesday in and out of Newark—a small fraction of the traffic there. Delta planned to cancel 57 flights.

The Port Authority of New York and New Jersey, which operates the Kennedy, LaGuardia and Newark Liberty airports, said it was lining up extra staff and snow removal equipment in the event of a heavy snowfall. Crews were prepared to work in 12-hour shifts if necessary, officials said.